The role of the Niemann-Pick type C2 protein as a sperm-binding protein in honeybees

The Niemann-Pick type C2 protein (NPC2) functions as a cholesterol modulator in the sperm membrane and enhances sperm physiological functions in vertebrates and some invertebrates. In insects, NPC2 is involved in lipid metabolism, immune response signaling, and chemical communication. However, the role of NPC2 as a sperm-associated protein in insect sperm remains to be elucidated. This study identified a novel function of the Apis mellifera NPC2 protein (AmNPC2) as a sperm-binding protein. Immunoassays and binding assays using recombinant AmNPC2 protein and its specific antibody revealed that AmNPC2 is expressed in the testes of drones and is localized on the sperm surface as a sperm-binding protein. Furthermore, a reduction in AmNPC2 levels on the sperm surface decreased sperm viability. AmNPC2 also appeared to play a protective role in maintaining sperm viability under both oxidative and temperature stress conditions. Our findings indicate that AmNPC2 is a sperm-binding protein that enhances the viability of honeybee sperm.